VARDA: looking into Earth’s past climates, one layer at the time

Sediments offer a unique window to look into Earth’s climate history. Varves are a specific type of sediments often found in lakes, where the annual to seasonal variations in the surrounding environment are well preserved. In this blog, we present a new tool allowing to precisely synchronise varved records and compare them to each other in order to advance our understanding of large-scale climatic changes and their local responses.

Written together with Arne Ramisch, Alexander Brauser and Achim Brauer (GFZ-Potsdam)

Nature offers several archives that record past climatic changes on Earth, such as trees, ice cores, stalactites or sediments. With sediments, one of the main challenges is the temporal resolution: because the deposition of sediments is quite slow, we can look far back into Earth’s history but these records are often very coarse and not continuous. Since we particularly want to understand climate changes that occur on timescales relevant to humans, we are on the look-out for sediments where we can check what happened year after year.

Dead_Sea_aad
Figure 1: Varves from the Dead Sea (Israel/Jordan). The annual cycles are composed of alternating pale carbonate (aragonite) laminae precipitated during arid summers and dark laminae consisting of organic and clastic matter washed during winter floods (Neugebauer et al., 2014; Ben Dor et al., 2019).

These sediments are called “varves”

Generally, varved sediments are found in lakes, where the accumulation of particles is fast and where no burrowing animals (similar to the garden worms) disturb the sediment layers. Thin, commonly <1 mm layers of sediments are deposited at the lake bottom following seasonal variations: during the rainy season, water washes soil particles and plant remains from the catchment and lake shores; when it is windy, air blows some dust onto the lake; during spring and summer, algae grow at the surface of the lake and sink to the bottom; these plant remains are then eaten by bacteria, which use oxygen from the lake waters; when there is no more oxygen, some strange minerals might form… These conditions can be determined in the laboratory by looking very closely at the composition of the layers, for instance under the microscope.

Varves are a great way to reconstruct climatic changes over long periods of time (thousands of years) while looking at the scale of the season or year.

We can see how long it takes for a lake to go from one state to another or how long it takes to see changes in the temperature and weather conditions. What we are after is to understand how lakes and their catchments (say the vegetation around the lake) respond to large-scale climatic changes (say an increase in global atmospheric temperature) and how long it takes for the local environment to recover from such a change. It is something we want to know because our climate is changing on a global scale and it does affect our living space.

It is also very useful to know how larger features of the climatic system are recorded. For instance, we want to look at two places that are in two different climatic regimes but underwent a similar forcing (e.g., a change in global temperatures). Was place A more or less affected than place B? Do we see that large climatic phenomena like monsoon systems moved further inland or otherwise? In order to do that, we need to have 1) records with a yearly resolution and 2) a way to make sure that we look at the same years in all sediment records.

P33_4_15-25
Figure 2: Analyses of sediments from the Nile delta representing annual sedimentation cycle during the last Green Sahara phase (ca. 10 to 8 thousand years ago). From left to right: depth scale (in cm), scan of cross-polarized light of a thin slide, chemical scan showing the concentrations of titanium (Ti, blue) versus calcium (Ca, yellow) and relative proportion of Ti/Ca along the median axis of the thin section Ln(Ti/Ca). High values of Ln(Ti/Ca) indicates the occurrence of Nile floods (see previous blog).

And that’s when VARDA comes into play

Over the last decades, more and more climate records from varved sediments became available, which gives us accounts of climatic changes at many single locations. To bridge the gap to the hemispheric and global scales, we need a synthesis of climate information from varved lake sediments worldwide. That’s what VARDA is designed to do.

The VARDA (VARved sediments DAtabase) project aims to provide harmonised climate information from varved lakes covering the last glacial cycle (the last 120,000 years). To do so, researchers from the GFZ German Research Centre for Geosciences in Potsdam supported by the PalMod project, financed by the Federal Ministry of Education and Research of Germany, currently collect and revise published and unpublished measurement data of varved sediment profiles provided by the international research community. From a technical point of view, the data is stored in a state-of-the-art (Neo4j) graph database. Graph technology allows to build flexible data models and software tools that can be adapted to the contemporary needs of the research community. On the other hand, it enables users to quickly find climate-related lake data for their respective study area and period of interest. An online tool to access the revised data is provided via https://varve.gfz-potsdam.de/ with links to the original data sources.

The first version of VARDA offers chronological information from 95 lakes worldwide, like varve counts, age estimations from radiocarbon dating and tephra layers (fallouts from volcanic ashes). A set of climate proxies and a data submission tool will be added in the next version of VARDA (scheduled in early 2021). The technical environment of VARDA is continuously improved, with tools for data aggregation and visualization on their way to forthcoming versions.

VARDA will help paleoclimatologists from all over the world to compare their climatic records to those in the database, grow the corpus of varved records for others to use and thereby advance our understanding of the complex climatic machinery of our planet.

VARDA-key-figure
Figure 3: Records available in version 1.0 of VARDA
Screenshot 2020-10-20 at 15.50.50
Figure 4: Screenshot of the landing page of the VARDA database

Lexicon

  • Climate proxy: a physical, geo- or biochemical measurement which represents specific climatic conditions. For instance, stable isotopes of oxygen in fossil organisms (e.g., the shells of small crustaceans called ostracods) represent the temperature of the lake water body. In Figure 2, the Ti/Ca ratio allows to detect past floods of the Nile River.
  • Forcing: Change in a (generally large-scale) parameter that will force other (more local) parameters to vary. In climate science it often refers to the changes in the Earth’s orbit (the forcing) that affect the repartition of solar energy at the surface of the Earth, thereby modulating the size of the ice caps, monsoonal systems and the atmospheric temperature (the responses).
  • Paleoclimatologist: could be described as an archeologist of the climate. They used different types of archives to reconstruct past changes in climate (temperature, winds, vegetation…) in order to better understand how fast, how much and generally all the “how” and “why” questions one can think of about the climatic system of the Earth.
  • Temporal resolution: in climate records, it refers to the time span between two observations. It varies from thousands of years to the season depending on how fast the sediments accumulate and how fine we can make our observations. For varved sediments, we measure and observe at a scale smaller than a millimetre to assess physical, chemical or biological parameter changes in annual to seasonal resolution (see figure 2).

References

Ben Dor, Y., Neugebauer, I., Enzel, Y., Schwab, M.J., Tjallingii, R., Erel, Y., Brauer, A., 2019. Varves of the Dead Sea sedimentary record. Quaternary Science Reviews 215, 173–184. 

Neugebauer, I., Brauer, A., Schwab, M.J., Waldmann, N.D., Enzel, Y., Kitagawa, H., Torfstein, A., Frank, U., Dulski, P., Agnon, A., Ariztegui, D., Ben-Avraham, Z., Goldstein, S.L., Stein, M., 2014. Lithology of the long sediment record recovered by the ICDP Dead Sea Deep Drilling Project (DSDDP). Quaternary Science Reviews 102, 149–165. 

Want to know more? 

Description of the VARDA Database in a ESSD Paper

PAGES Varve working group

Brauer, A., Dulski, P., Mangili, C., Mingram, J., Liu, J., others, 2009. The potential of varves in high-resolution paleolimnological studies. Pages News 17, 96–98.

Schimmelmann, A., Lange, C.B., Schieber, J., Francus, P., Ojala, A.E.K., Zolitschka, B., 2016. Varves in marine sediments: A review. Earth-Science Reviews 159, 215–246. 

Zolitschka, B., Francus, P., Ojala, A.E.K., Schimmelmann, A., 2015. Varves in lake sediments – a review. Quaternary Science Reviews 117, 1–41.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

Create a free website or blog at WordPress.com.

Up ↑

%d bloggers like this: